Saw a low-flying rental... let the FBO know?

Driving home today I saw a 172 that I've flown many times heading toward me
slowly at about 500'. This is in SoCal, so it's obviously a densely packed
urban area. In fact I pulled over and turned my car off so I could hear if
the Cessna was having engine trouble.

I'm debating whether to inform the FBO that someone was flying one of their
planes well below the legal limit. I know exactly which plane it was
because it has a distinctive paint job, patterned after the state flag of
Texas. Would I just be a rat or do you think it's a legitimate safety
hazard that they should know about?